1. Technical Field
This invention relates to a photoelectric encoder for detecting a position, angle, speed, angular speed, etc., a scale therefor and a method for manufacturing the same.
2. Related Art
Generally, the photoelectric encoder includes a main scale, an index scale, a light source, and a light receiving section. The main scale extends along a measuring axis. The index scale is provided so as to move relatively to the main scale. The light source projects light onto the main scale through the index scale. The light receiving section receives, through the index scale, the light emitted from the light source and reflected from the main scale. The main scale and the index scale have scale divisions formed with a predetermined pitch, respectively. By optically detecting changes in the phase of the scale divisions formed on both scales by relative movement therebetween, the position of the index scale relative to the main scale is detected.
Meanwhile, where it is necessary to from a very long scale as the main scale in the use of a large scale measuring machine for e.g. vehicle body measurement, it is difficult to form the main scale of a glass scale. For this reason, in this kind of use, a metallic scale formed of a stainless tape has been used. As a method for manufacturing the metallic scale, as disclosed in e.g. JP-T-2003-512611 (paragraphs 0010 to 0012, FIGS. 1, 2 and 5) (which is hereinafter referred as Patent Reference 1), the method is known comprising the steps of forming dips and bumps on the surface of a tape-like scale when the scale passes between rollers with uneven contours and of unifying the height of the bumps by an additional processing of passing the tape between flat rollers. Further, as disclosed in WO03/061891 (page 7, line 29 to page 8, line 7, FIG. 2) (which is hereinafter referred as Patent Reference 2), there is a known method for manufacturing the scale by making scale divisions on a stainless ribbon by laser light.
However, the scale disclosed in Patent Reference 1 can be manufactured easily but the accuracy of each bump and each dip formed on the scale is low. Therefore, this scale can be used in the use of assuring the accuracy by an averaging effect of the detection signal outputted from the light receiving section by receiving the light from plurality sets of grids as detection area on the scale. However, this scale cannot be used for a displacement measuring machine requiring high accuracy.
Further, the scale disclosed in Patent Reference 2 provides a preferable accuracy of the scale divisions. However, the apparatus for manufacturing this scale becomes a large-scale so that the production cost of the scale is high. Further, in this scale, the area of scale divisions constitute a non-reflecting face. Therefore, light is absorbed in this area and the quantity of light received by the light receiving section is lowered. In the case of such a kind of ribbon-shaped scale, the scale gives undulation so that a change of gap between the scale and the light receiving section is large and so an error is likely to occur with a minute signal. Thus, if the scale divisions constitute a light absorbing area, the S/N is lowered owing to shortage of signal strength.